The advantages of acrylic polymers as viscoelastic bases for pressure-sensitive adhesives are well known in the art. U.S. Pat. No. Re. 24,906 (Ulrich) cites many examples of these products. Initially, such compositions were made via solution polymerization. However, such methods of polymerization involved the use of large amounts of organic solvents. This was undesirable for both safety and economic reasons. Further, the necessity of an excess of such solvents in order to render the polymers transferable between containers drastically limited the handleability and transportability of the polymers.
Various methods of suspension or emulsion polymerization for copolymer pressure-sensitive adhesives have been disclosed in the art. Emulsion polymerization uses water as the medium, dissipates the exotherm due to the heat of polymerization, and is extremely safe. Further since the polymerization takes place within the micelle, the molecular weight is easily controlled, and the resulting emulsion is safer to handle. However, the pressure-sensitive adhesives made via emulsions differed in performance properties and/or coatablity from those made via solutions depending on the techniques employed, i.e., emulsion-formed adhesives frequently exhibited lessened adhesion.
U.S. Pat. No. 3,691,140 discloses pressure-sensitive adhesive microspheres having average diameters of from 5 micrometers to about 150 macrometers formed via emulsion polymerization. These microspheres are designed to maintain their beaded configuration when coated. This results in a porous coating rather than a continuous coating. Upon standing, the microspheres separate into two or more phases.
Suspension polymerization of pressure-sensitive adhesives has been attempted, but keeping the polar monomers such as acrylic acid in the organic phase long enough for polymerization with the alkyl acrylate monomers has been difficult. Japanese Laid-Open patent application No. 57-42778, published Mar. 10, 1912, discloses a pearl-shaped pressure-sensitive adhesive polymer obtained by suspension polymerization of optional methacrylic acid in alkyl acrylates and a specially formulated dispersing agent consisting of a copolymer made up of from 80-99.5% of a hydrophilic monomer and from 0.5-20% of a hydrophobic monomer. Conventional dispersing agents are disclosed to cause the resulting pearl-shaped polymers to block or agglomerate during polymerization. Furthermore, it is disclosed that even the specially formulated dispersing agents described may lower the adhesive properties of the copolymer.
German Patent No. 24 55 133, published Mar. 4, 1982, discloses suspension polymerization of acrylate monomers into bead-shaped polymers. The polymer beads are coated with a crosslinking agent after polymerization to render them sensitive to radiation. No water-soluble polar monomers are used in the adhesives. The beads themselves need not even be tacky; low adhesion values are disclosed as is the addition of conventional tackifying agents.
U.S. Pat. No. 3,786,116 discloses use of macromolecular monomers (macromers) in water-based polymerization systems. No pressure-sensitive adhesive systems are disclosed.
U.S. Pat. No. 4,554,32discloses hot melt pressure-sensitive adhesives formed by copolymerization of macromers in alkyl acrylates to improve shear holding properties. Emulsion polymerization is disclosed, but not exemplified or preferred; all examples utilize solution polymerization.
U.S. Pat. No. 4,551,388 similarly incorporates from 1% to 30% methacrylate macromonomer by copolymerization.
U.S. patent application Ser. No. 895,323, filed Aug. 11, 1986, assigned to the assignee of the present application, discloses solvent polymerization of pressure-sensitive adhesives comprising zinc carboxylates and methacrylate macromonomers.